Pulse laser systems consisting of an oscillator and a high power fiber amplifier have been developed and are well known in the art. See, for example, U.S. Pat. Nos. 6,208,458; 6,181,463; and 5,696,782, all of which are assigned to the same assignee as the present invention and all of which are included herein by reference for all that they teach. Conventional systems of this type typically operate at a fixed pulse width and repetition rate. However, systems with variable pulse widths and repetition rates have also been developed. For example, see U.S. Pat. Nos. 6,347,007; 6,335,941; and 6,433,306 to Grubb et al. For certain applications, it is desired that the output pulse energy of these types of systems be maintained at a constant value as the pulse width and repetition rate of the pulse source is varied.
The gain of the amplifier system depends on the rate at which energy is stored into, and depleted from, the doped fiber. Therefore, output pulse energy varies as a function of the rate of energy storage into the amplifier and the repetition rate of the seed, which extracts energy from the amplifier. A known method includes adjusting the gain of the amplifier by adjusting the power of the amplifier pump diode by changing its drive current as a function of the seed source pulse energy and repetition rate. A further alternative, which is suitable under some conditions, includes changing the pulse energy of the seed source by modulating the source directly or attenuating its input to the power amplifier.
The gain of the fiber amplifiers in these systems is a function of the wavelength of the pump diode as well as a function of the pump diode power. The optimum pump diode wavelength is a function of the material properties of the amplifier fiber. For example, in a Yb amplifier system, the optimum pump wavelength is approximately 976 nm for the shortest amplifier length. Further, the wavelength of the pump diode is a function of diode temperature. In most practical systems, and particularly in systems using amplifier pump diodes containing multiple diodes packaged in a single package, the actual diode temperature and emission wavelength will vary as a function of diode current, even if the package is nominally held at a constant temperature. This is so because the temperature sensor used to monitor the diode temperature is located a short distance from the actual diode. Therefore, to obtain further control over the output pulse energy it is necessary to counter wavelength drift of the pump diode by a more accurate and actively controlled mechanism.
Also influencing the gain of the fiber is the temperature of the fiber. One physical process for changing this gain is made possible because rare earth ions, such as Yb and Er, are actually quasi three-level lasing materials rather than four-level lasing materials. This means that at the lasing wavelength there is some absorption. The amount of absorption is determined by the Boltzman distribution of states, which is temperature dependent.
U.S. Pat. Nos. 5,867,305 and 5,933,271 to Waarts et al. disclose pulsing the pump laser diodes for the purpose of preventing the buildup of amplified noise between two fiber amplifiers for amplifying pulses. An improvement to these systems is to avoid the need for a gate synchronized to the seed pulse. However, the build-up time of amplified noise in such a system is the round-trip time of the total fiber amplifier length which is 10-100 ns and not the fluorescence lifetime which is 400 μsec to 10 msec as stated in the two references mentioned above. Under most conditions sufficient gain cannot be obtained in a fiber amplifier when the pump laser diode is pulsed for 10-100 ns.